In optical communication systems, a Mach-Zehnder pat is used as an optical modulator. The Mach-Zehnder part that utilizes the Mach-Zehnder interference is configured to have a pair of optical waveguides on a substrate having the electro-optical effect. Electric fields are applied to the optical waveguides by using electrodes in order to change the refractive indexes in the optical waveguides. This changes the difference in phase between the pair of optical waveguides. For example, the pair of optical waveguides is combined to yield the Mach-Zehnder interference, and a signal light with the optical strength being modulated is thus obtained.
In order to output a signal light modulated by QPSK (Quadrature Phase Shift Keying), there is known an optical modulator in which a secondary Mach-Zehnder part is connected to each of the pair of optical waveguides of a primary Mach-Zehnder part (see U.S. Pat. No. 7,272,271, for example).
In a case where the multiple Mach-Zehnder parts are integrated on the single substrate, there is known an art of forming a groove on the substrate so as to be located between the Mach-Zehnder parts in order to suppress crosstalk between these Mach-Zehnder parts (see Japanese Laid-Open Patent Application No. 2009-53444, for example). Further, there is known an art of forming a groove on the back surface of the substrate (see Japanese Laid-Open Patent Application No. 2001-174766, for example).